1. Field of the Invention
The present invention relates to a phase shift mask, a blank for a phase shift mask, and a method of manufacturing a phase shift mask.
2. Description of the Background Art
As the higher integration and miniaturization have been achieved in a semiconductor integrated circuit, miniaturization of the circuit pattern formed on a semiconductor substrate (hereinafter simply referred to as a wafer) has also been promoted.
As a basic technique for the pattern generation, photolithography is widely known among others. Although various development and improvement have been proceeded in the field, the dimension of the pattern has been still smaller, and the requirement for the resolution of the pattern has also been stronger.
According to the photolithography technique, a mask (original) pattern is transferred to a photoresist coating a wafer, and an underlying film to be etched is patterned using the transferred photoresist. At the time of transfer, the photoresist is developed. Through the development process, the photoresist of the type in which a portion exposed to light is removed is called appositive type photoresist, while the type in which a portion not exposed to light is removed is called a negative type.
Resolution limit R (nm) in the photolithography employing the demagnification exposure method is represented as EQU R=k.sub.1 .multidot..lambda./(NA)
where .lambda. is wavelength (nm) of the light used, NA is numerical aperture of a lens, and k.sub.1 is a constant dependent on the resist process.
As can be understood from above equation, in order to improve the resolution limit R to obtain a fine pattern, the values k.sub.1 and .lambda. should be smaller, and the value NA should be larger. In other words, what is to be done is to reduce the constant dependent on the resist process and to shorten the wavelength and to increase NA.
However, improvement of light source or the lens is technically difficult, and depth of focus .delta. of the lens (.delta.=k.sub.2 .multidot..lambda./(NA).sup.2) might become shallower by shortening the wavelength and increasing NA, thus causing the deterioration of the resolution.
In view of this, studies of miniaturization of the pattern by improving not the light source or the lens but the photomask are proceeded. Lately, a phase shift mask has been attracting much attention as a photomask allowing improvement of the resolution of the pattern. The structure and principle of such a phase shift mask will be hereinafter described in comparison with an ordinary photomask. The description below will be directed to a phase shift mask of the Levenson system.
FIGS. 27A, 27B, and 27C respectively show a cross section of a mask, electric field on the mask, and light intensity on the wafer when an ordinary photomask is used. With reference to FIG. 27A, the ordinary photomask is structured to have a metal mask pattern 403 formed on a glass substrate 401. In the electric field on such an ordinary photomask, the pulse is modulated spatially by metal mask pattern 403 as shown in FIG. 27B.
Referring to FIG. 27C, if the pattern has smaller dimension, the exposure light transmitted through the photomask extends into a non-exposed region (a region where the transmission of the exposure light is blocked by metal mask pattern 403) on the wafer due to the diffraction effect of the light. The light is thus directed to the region not to be exposed on the wafer, resulting in deterioration of the contrast of the light (difference of the light intensity between an exposed region and a non-exposed region on a wafer). The resolution is degraded and transfer of a fine pattern becomes difficult.
FIGS. 28A, 28B and 28C respectively show a cross section of a mask, electric field on the mask, and light intensity on a wafer when a phase shift mask of the Levenson system is used. With reference to FIG. 28A, an optical member called a phase shifter 405 is provided on an ordinary photomask.
More specifically, chromium mask pattern 403 is formed on glass substrate 401 to provide an exposure region and a light blocking region, and phase shifter 405 is formed at every other exposure region. Phase shifter 405 has a function of shifting the phase of the transmitted light by 180.degree..
Referring to FIG. 28B, in the electric field on the mask generated by the light transmitted through the phase shift mask, the phases are alternately inverted by 180.degree. since phase shifters 405 are provided at every other exposure region. As described above, adjacent exposed regions have opposite phases of light, so that beams of light are cancelled with each other due to the interference of light in the portions where reverse-phased beams of light are overlapped.
As a result, as shown in FIG. 28C, the intensity of the light becomes weak in the boundary portion between the exposed regions, then sufficient difference of light intensity between the exposed region and the non-exposed region on the wafer can be ensured. The improvement of the resolution is thus possible to allow the transfer of a fine pattern.
The phase shift mask of the Levenson system explained above has a superior resolution in view of this principle, and such system is considered as the most favorable system from the standpoint of resolving power among other various kinds of phase shift masks.
FIG. 29 schematically illustrates the cross section of the structure of the conventional phase shift mask of the Levenson system. With reference to FIG. 29, the conventional phase shift mask is provided with a transparent substrate 501 formed of quartz, an etching stopper layer 503 formed of SnO film, a phase shifter 505 formed of SiO.sub.2 film, and a light blocking film 507 formed of Cr film.
Etching stopper film 503 is formed on transparent substrate 501. Phase shifter film 505 is formed to cover a first light transmitting region Ta and a light blocking region S and to expose a second light transmitting region Tn on etching stopper film 503. Light blocking film 507 is formed to cover transparent substrate 501 in light blocking region S located between adjacent first and second light transmitting regions Ta and Tn.
Generally, upon the exposure in the transfer process, the exposure light of uniform intensity is directed to the phase shift mask from the side of transparent substrate 501. The respective phases of the exposure light transmitted through the first light transmitting region Ta and the second light transmitting region Tn are inverted by 180.degree.. The transmitted light with its phases inverted from each other is directed to the photoresist, and the pattern having a shape corresponding to light transmitting regions Ta and Tn is provided on the photoresist through the development.
If the first and the second light transmitting regions Ta and Tn have the same opening dimension, the same amount of light should be transmitted through each of light transmitting regions Ta and Tn in order to form a pattern of a photoresist of uniform dimension. However, in the conventional phase shift mask, the films are not appropriately structured in the first and the second light transmitting regions Ta and Tn, and the amount of light transmitted through the first and the second light transmitting regions Ta and Tn is not necessarily uniform.
Further, SnO used for etching stopper film 503 has a large refractive index. Therefore, the amount of light transmitted through the first and the second light transmitting regions Ta and Tn will be different even if the opening dimension of the first and the second transmitting regions Ta and Tn is large enough to cancel the effect of the shape generated by the processing. The pattern formed on the photoresist accordingly has different dimension as described above.
An invention aiming at overcoming this problem is shown in Japanese Patent Laying-Open No. 7-159971.
FIG. 30 schematically shows a cross section of the structure of the phase shift mask shown in the laid-open application. With reference to FIG. 30, a phase shifter film 205 is formed on a transparent substrate 201 with an etching stopper film 203 formed of alumina (Al.sub.2 O.sub.3) interposed, and a light blocking film 207 is provided thereon to cover light blocking region S.
This approach aims at providing the same amount of light transmitted though the first and the second light transmitting regions Ta and Tn by adjusting the film thickness and the refractive index of phase shifter film 205.
In this structure, double layers of etching stopper layer 203 and phase shifter film 205 are provided on transparent substrate 201 in the first light transmitting region Ta. The amount of light transmitted through the first light transmitting region Ta is determined depending on the interaction between etching stopper layer 203 and phase shifter film 205. Therefore, the film thickness and the like are required to be adjusted for both of etching stopper layer 203 and phase shifter film 205 in order that the same amount of light is transmitted through the first and the second transmitting regions Ta and Tn.
As only phase shifter film 205 is considered in the technique shown in the patent, the amount of the light transmitted through the first and the second light transmitting regions Ta and Tn cannot be adjusted to be uniform.
A structure is disclosed in Japanese Patent Laying-Open No. 7-72612 in which etching stopper layer 203 of the structure shown in FIG. 30 is removed in the second light transmitting region Tn as shown in FIG. 31.
In the structures shown in FIGS. 30 and 31, a problem arises because of alumina used for etching stopper layer as described below.
Sputtering method is generally employed when a film is formed of alumina. In this case, metal is used as a target, and the sputtering ambient includes O.sub.2 (Oxygen). A part of the target becomes insulated due to the ambient, and the discharge during the sputtering becomes unstable. Local arcing current is accordingly generated to cause melting and scattering of a portion of the target.
In the ordinary sputtering, atoms or molecules are deposited on a transparent substrate. In this case, a relatively large melted material drops on the transparent substrate. When such large melt drops on the transparent substrate, the large melt may repel a photoresist deposited thereon. When an aluminum film is etched, the large melt of alumina is difficult to be etched and removed completely. Further, the large melt of alumina makes it impossible to obtain a phase shift mask of higher resolution since the phase in the region containing the large melt of alumina is different from that in the other region.
Although a film can be formed of alumina by CVD (Chemical Vapor Deposition) method, temperature of 1000.degree. C. or more is required. At such a high temperature, quartz as a material of transparent substrate 501 could distort, then a phase shift mask of higher resolution cannot be obtained when alumina is deposited by CVD method.